淬火冷却工艺对F460钢调质厚板组织与性能的影响

Influence of quenching cooling process on microstructure and properties of quenched and tempered F460 steel thick plate

对120 mm厚的F460钢调质厚板采用相同的淬火回火温度,不同的淬火冷却速度处理,之后对钢板进行组织与性能对比,寻找该钢种的最佳热处理工艺。采用2℃/s冷速进行冷却的钢板,回火后强度最高,但是冲击性能不佳;适当降低淬火冷却速度后,钢板回火后强度有一定下降,但是冲击性能得到明显提升;继续降低淬火冷却速度,钢板回火后强度进一步下降,但是冲击性能提升有限。经组织分析,2℃/s冷速进行冷却淬火时,钢板回火后的组织为铁素体+贝氏体组织,组织中主要是贝氏体;冷却速度降低以后,钢板回火后组织为铁素体+退化珠光体组织,铁素体含量的增加,有利于钢板韧性的提升,残留奥氏体回火后形成的珠光体组织比较细小,能有效保证钢板的强度。通过对钢板的连续冷却转变曲线进行分析,钢板在冷却过程中先开始进行铁素体相变,溶质元素向奥氏体迁移。在钢板冷速较快时,铁素体中的碳化物迁移较少,奥氏体低温时转变成马氏体或者贝氏体;在钢板冷速较慢时,碳化物迁移到奥氏体内,提高奥氏体稳定性并保留到室温,形成残留奥氏体。残留奥氏体在后续的高温回火过程中,转变成珠光体。块状转变形成的铁素体组织与回火过程中形成的细小珠光体有利于钢板的强韧性匹配。

Microstructure and properties of 120 mm thick F460 steel quenched and tempered plate with the same quenching and tempering temperature and different quenching cooling rates were compared to find the best heat treatment process of the tested steel. The steel plate quenched with the 2 ℃/s cooling rate has the highest strength after tempering, but the impact property is poor. After rationally reducing the quenching cooling rate, the strength of the steel plate after tempering decreases to a certain extent, but the impact toughness is significantly improved. Further reducing the quenching cooling rate further reduces the strength of the steel plate after tempering, but the improvement of impact property is limited. The microstructure analysis shows that when the steel plate is quenched at 2 ℃/s cooling rate, the microstructure after tempering is ferrite and bainite, and bainite is mainly in the microstructure. When the cooling rate is reduced, the structure of the steel plate after tempering is ferrite and degenerate pearlite. The increase of ferrite content is conducive to the improvement of toughness of the steel plate. The pearlite structure formed after tempering from retained austenite is relatively small, which can effectively ensure the strength of the steel plate. By analyzing the continuous cooling transformation curve of the steel plate, the steel plate begins to undergo ferrite transformation firstly and solute elements migrate to austenite during cooling. When the cooling rate of steel plate is fast, the carbides in ferrite migrate less, and austenite transforms into martensite or bainite at lower temperature. When the cooling rate of steel plate is slow, carbides migrate into austenite, which improves the stability of austenite and retains it to room temperature to form retained austenite. Retained austenite is transformed into pearlite in the subsequent high-temperature tempering process. The ferrite structure formed by massive transformation and the fine pearlite formed during tempering are conducive to the strength toughness matching of the steel plate.